Treatment of electrodeposition bath

ABSTRACT

This invention relates to a method of rinsing articles coated by an electrodeposition process in such a manner that at least a portion of the rinse water returns to the electrodeposition bath, thereby reducing dragout losses. The percentage of solids are controlled by utilizing a membrane-controlled selective separation process such as an ultrafiltration process to control the electrodeposition bath composition.

United States Patent Le Bras et al.

[15] 3,663,397 [451 May 16, 1972 TREATMENT OF ELECTRODEPOSITION BATH Louis R. Le Bras, Gibsonia; John S. 0strowski, Pittsburgh, both of Pa.

Assignee: PPG Industries, Inc., Pittsburgh, Pa. Filed: Sept. 14, 1970 Appl. No.: 71,742

Inventors:

Related US. Application Data Continuation-in-part of Ser. No. 883,584, Dec. 9, 1969, abandoned.

US. Cl ..204/181 Int. Cl. ..BOIk 5/02, C23b 13/00 Field of Search ..204/181 [56] References Cited UNITED STATES PATENTS 3,355,373 11/1967 Brewer et a1 204/181 3,556,970 l/1971 Wallace et a1. ..204/181 FOREIGN PATENTS OR APPLICATIONS 1,071,458 6/1967 Great Britain ..204/181 Primary Examiner-Howard S. Williams Attorney-Chisholm and Spencer [51] ABSTRACT This invention relates to a method of rinsing articles coated by an electrodeposition process in such a manner that at least a portion of the rinse water returns to the electrodeposition bath, thereby reducing dragout losses. The percentage of solids are controlled by utilizing a membrane-controlled selective separation process such as an ultrafiltration process to control the electrodeposition bath composition.

5 Claims, 2 Drawing Figures Patented May 16, 1972 INVENTORS Lows 49 466x115 JOHN 5. OSTQOWSKI w ATTORNEY$ TREATMENT OF ELECTRODEPOSITION BATH CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of copending Application Ser. No. 883,584, filed Dec. 9, 1969 now abandoned.

STATE OF THE ART Electrodeposition has become a widely commercially accepted industrial coating technique. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run or wash off during baking. Virtually any conductive substrate may be coated by electrodeposition. Most commonly employed are metal substrates, including metals such as iron, steel, copper, zinc, brass, tin, nickel, chromium, and aluminum, as well as other metals and pretreated metals. Impregnated paper or other substances rendered conductive under the conditions of the coating process may also be employed as substrates.

In the electrodeposition process, the articles to be electrocoated are immersed in an aqueous dispersion of a solubilized, ionized, film-forming material such as a synthetic organic vehicle resin. An electric current is passed between the article to be coated, serving as an electrode and a counter electrode to cause electrodeposition of a coating of the vehicle resin on the articles. The articles are then withdrawn from the bath, usually rinsed and then the coating either air-dried or baked in the manner of a conventional finish.

In the electrodeposition process, when the article has been coated and is being withdrawn from the coating bath, a portion of the coating material which is not electrocoated to the article, but merely adherent thereto, is withdrawn with the article. This material is commonly called dragout. This dragout is generally rinsed from the article, leaving an adherent electrocoated film which is then dried or baked in a conventional manner. This dragout represents waste material and reduces the efficiency of the system. Further, the rinse water containing the dragout comprises a waste disposal problem. Typically, rinsing is conducted with either tap water or deionized water.

DESCRIPTION OF THE INVENTION It has now been found that in conjunction with a selective membrane, rinsing to remove dragout from an electrodeposited article may be accomplished by returning all or a portion of the rinse material to the electrodeposition bath, a compensatory amount of water, or water and low molecular weight solute being either continuously or intermittently removed from the bath by use of a selective membranemoderated process, such as ultrafiltration. Various advantages are derived from this technique. The chief of these is that economics from low dragout losses can be achieved without unduly changing the composition of the bath. Further, the return of dragout to the system substantially reduces environmental pollution by the total electrodeposition system.

The control of an electrodeposition bath by an ultrafiltration process has been described in copending Application Ser. No. 814,789, filed Apr. 9, 1969. In the ultrafiltration process, excellent control of a bath composition and removal of objectionable accumulated materials has been achieved by a selective filtration process, that is, a process which selectively removes low molecular weight materials from the bath composition. The selective filtration process removes excess counter-ion and thus serves as a method of conventional bath control but, in addition, this method further removes other excess materials or contaminants from the bath, thus permitting more complete control over bath constituents than heretofore has been possible.

The selective filtration process is an ultrafiltration process which separates materials below a given molecular weight size from the electrodeposition bath. With properly selected membranes, this treatment does not remove any product or desirable resin from the paint in the tank but does remove anionic,

cationic and nonionic materials from the paint in the water phase of the paint. Thus, for example, it is possible to remove amines, alkaline metal ions, phosphates, chromates, sulfates, solvents and dissolved carbon dioxide, among others.

Ultrafiltration may be defined as a method of concentrating solute while removing solvent, or selectively removing solvent and low-molecular weight solute from a significantly higher molecular weight solute. From another aspect, it is a process of separation whereby a solution containing a solute of molecular dimensions significantly greater than the solvent is depleted of solute by being forced under a hydraulic pressure gradient to flow through a suitable membrane. The first definition is the one which most fittingly describes the term ultrafiltration as applied to an electrodeposition bath.

Ultrafiltration thus encompasses all membrane-moderated, pressure-activated separations wherein solvent or solvent and smaller molecules are separated from modest molecular weight macromolecules and colloids. The term ultrafiltration is generally broadly limited to describing separations involving solutes of molecular dimensions greater than about 10 solvent molecular diameters and below the limit of resolution of the optical microscope, that is, about 0.5 micron. In the present process, water is considered the solvent.

The principles of ultrafiltration and filters are discussed in a chapter entitled Ultrafiltration in the Spring, 1968, volume of ADVANCES IN SEPARATIONS AND PURIFICATIONS, E. S. Perry, Editor, John Wiley & Sons, New York, as well as in CHEMICAL ENGINEERING PROGRESS, Vol. 64, December 1968, pages 31 through 43, which are hereby incorporated by reference.

The basic ultrafiltration process is relatively simple. Solution to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in a cell, in contact with an appropriate filtration membrane supported on a porous support. Any membrane or filter having chemical integrity to the system being separated and having the desired separation characteristics may be employed. Preferably, the contents of the cell should be subjected to at least moderate agitation to avoid accumulation of the retained solute on the membrane surface with the attendant binding of the membrane. Ultrafiltrate is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent or solvent plus dissolved low molecular weight solute is removed. A suitable apparatus for conducting ultrafiltration is described in US. Pat. No. 3,494,465, which is hereby incorporated by reference.

There are two types of ultrafiltration membrane. One is the microporous ultrafilter, which is a filter in the traditional sense, that is, a rigid, highly voided structure containing interconnected random pores of extremely small average size. Through such a structure, solvent (in the case of electrodeposition, water) flows essentially viscously under a hydraulic pressure gradient, the fiow rate proportional to the pressure difference, dissolved solutes, to the extent that their hydrated molecule dimensions are smaller than the smallest pores within the structure, will pass through, little impeded by the matrix. Larger size molecules, on the other hand, will become trapped therein or upon the external surfaces of the membrane and will thereby be retained. Since the microporous ultrafilters are inherently susceptible to internal plugging or fouling by solute molecules whose dimensions lie within the pore size distribution of the filter, it is preferred to employ for a specific solute a microporous ultrafilter whose means pore size is significantly smaller than the dimensions of the solute particle being retained.

In contrast, the diffusive ultrafilter is a gel membrane through which both solvent and solutes are transported by molecular diffusion under the action of a concentration or activity gradient. In such a structure, solute and solvent migration occurs via random thermal movements of molecules within and between the chain segments comprising the polymer network. Membranes prepared from highly hydrophilic polymers which swell to eliminate standard water are the most useful diffusive aqueous ultrafiltration membranes. Since a diffusive ultrafilter contains no pores in the conventional sense and since concentration within the membrane of any solute retained by the membrane is low and time-independent, such a filter is not plugged by retained solute, that is, there is no decline in solvent permeability with time at a constant pressure. This property is particularly important for a continuous concentration or separation operation. Both types of filters are known in the art.

The presently preferred ultrafilter is an anisotropic membrane structure such as illustrated in FIG. 1. This structure consists of an extremely thin, about one-tenth to about 10 micron layer, of a homogeneous polymer 1 supported upon a thicker layer of a microporous open-celled sponge 2, that is, a layer of about 20 microns to about 1 millimeter, although this dimension is not critical. If desired, this membrane can be further supported by a fibrous sheet, for example, paper, to provide greater strength and durability. These membranes are used with a thin film or skin side exposed to the high pressure solution. The support provided to the skin by the spongy substrate is adequate to prevent film rupture.

Membranes useful in the process are items of commerce and can be obtained by several methods. One general method is described in Belgian Pat. No. 721,058. This patent describes a process which, in summary, comprises (a) forming a casting dope of the polymer in an organic solvent, (b) forming a film of the casting dope, and (c) preferentially contacting one side of said film with a diluent having high compatibility with the casting dope to effect precipitation of the polymer immediately upon coating the casting film with the diluent.

The choice of a specific chemical composition for the membrane is determined to a large extent by its resistance to the chemical environment. Membranes can be typically prepared from thermoplastic polymers such as polyvinyl chloride,

polyacrylonitrile, polysulfones, poly(methyl methacrylate),.

polycarbonates, poly(n-butyl methacylates), nylons, as well as a large group of copolymers formed from any of the monomeric units of the above polymers, including Polymer 360, a polysulfone copolymer. Cellulose materials such as cellulose acetate may also be employed as membrane polymers.

Some examples of specific anisotropic membranes operable in the process of the invention include: Diaflow membrane ultrafilter PM-30, the membrane chemical composition of which is a polysulfone copolymer, Polymer 360, and which has the following characteristics:

Solute Retention Characteristics Flux Molecular Re- (gal./sq.ft./day at Solute weight tention 30 psi/1.0% solute Cytochrome C 12,600 50 100 o: Chymotripsinagen 24,000 22 Ovalbumin 45,000 45 This membrane is hereinafter referred to as Membrane B.

Dorr-Oliver BPA type membrane, the membrane chemical composition of which is phenoxy resin (polyhydroxy ether) and which has the following permeability characteristics:

Flux

Molecular Re- (gal./sq.ft./day at Solute weight tention 30 psi, 1.0% solute) Cytochrome C 12,600 50 30 This membrane is hereinafter referred to as Membrane C."

The microporous ultrafilters are generally isotropic structures, thus flow and retention properties are independent of flow direction. it is preferred to use an ultrafilter which is anisotropic in its microporous membrane structure, FIG. 2. In such a membrane, the pore size increases rapidly from one face to the other. When the fine textured side 4 is used in contact with the feed solution, this filter is less susceptible to plugging since a particle which penetrates the topmost layer cannot become trapped in the membrane because of the larger pore size 5 in the substrate.

The process of the invention may be operated as either a batch or a continuous process. In batch selective filtration or batch ultrafiltration, a finite amount of material is placed in a cell which is pressurized. A solvent and lower molecular weight solutes are passed through the membrane. Agitation is provided by a stirrer, for example, a magnetic stirrer. Obviously, this system is best used for small batches of material. in a process requiring continuous separation, a continuous selective filtration process is preferred. Using this technique, material is continuously recirculated under pressure against a membrane or series of membranes through interconnecting flow channels, for example, spiral flow channels.

Likewise, the ultrafiltration process may be conducted as either a concentration process or a diafiltration process. Concentration involves removing solvent and low molecular weight solute from an increasingly concentrated retentate. Filtration flow rate will decrease as the viscosity of the concentrate increases. Diafiltration, on the other hand, is a constant volume process whereby the starting material is connected to a reservoir of pure solvent, both of which are placed under pressure simultaneously. Once filtration begins, the pressure source is shut off in the filtration cell, and, thus, as the filtrate is removed, an equal volume of new solvent is introduced into the filtration cell to maintain the pressure balance.

The configuration of the filter may vary widely and is not limiting to the operation of the process. The filter may be in the form of sheets, tubes or hollow fiber bundles, among other configurations.

Under ideal conditions, selected low molecular weight solutes would be filtered as readily as solvent and their con centration in the filtrate is equal to that in the retentate. Thus,

constant and the mathematical relationship is as follows:

, 9. Y.- i V0 where C is the initial solute concentration, C, is the final solute concentration of the retentate, V, is the volume of solute delivered to the cell (or the volume of the filtrate collected), and V is the initial solution volume (which remains constant).

Electrodepositable compositions, while referred to as solubilized," in fact, are considered a complex solution, dispersion or suspension or combination of one or more of these classes in water, which acts as an electrolyte under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances and perhaps in most the vehicle resin is a dispersion which may be called a molecular dispersion of molecular size between a colloidal suspension and a true solution.

The typical industrial electrodepositable composition also contains pigments, crosslinking resins and other adjuvants which are frequently combined with the vehicle resin in a chemical and a physical relationship. For example, the pigments are usually ground in a resin medium and are thus wetted" with the vehicle resin. As can be readily appreciated then, an electrodepositable composition is complex in terms of the freedom or availability with respect to removal of a component or in terms of the apparent molecular size of a given vehicle component.

As applied to the process of this invention, ultrafiltration comprises subjecting an electrodepositable composition, especially after it has been employed in a coating process, which inherently causes contaminants and other low molecular weight materials to accumulate in the bath, such as metal pretreatment chemicals, water, absorbed CO (either dissolved or, more likely, combined as an aminic salt or carbonate), neutralizing agent, organic solvent and ions such as chromate, phosphate, chloride and sulfate, for example, to an ultrafiltration process employing an ultrafilter, preferably a diffusive membrane ultrafilter selected to retain the solubilized vehicle resin while passing water and low molecular weight solute, especially those with a molecular weight below about 500. As previously indicated, the filters discriminate as to molecular size rather than actual molecular weight, thus, these molecule weights merely establish an order of magnitude rather than a distinct molecular weight cut-ofi'. Likewise, as previously indicated, the retained solutes may, in fact, be colloidal dispersions or molecular dispersions rather than true solutes.

In practice, a portion of the electrodepositable composition may be continuously or intermittently removed from the electrodeposition bath and passed under pressure created by a pressurized gas or by means of pressure applied to the contained fluid in contact with the ultrafilter. Obviously, if desired, the egress side of the filter may be maintained at a reduced pressure to create the pressure difference.

The pressures necessary are not severe. The maximum pressure, in part, depends on the strength of the filter. The minimum pressure is that pressure required to force water and low molecular weight solute through the filter at a measurable rate. With the presently preferred membranes, the operating pressures are between about and 150 p.s.i., preferably between about 25 and 75 p.s.i. Under most circumstances, the ultrafilter should have an initial flux rate, measured with the composition to be treated of at least about 3 gal./sq.ft./day (24 hours) and preferably at least about 4.5 gal./sq.ft./day.

As previously indicated, the bath composition should be in motion at the face of the filter to prevent the retained solute from impeding the flow through the filter. This may be accomplished by mechanized stirring or by fiuid flow with a force vector parallel to the filter surface.

The retained solutes comprising the vehicle resin are then returned to the electrodeposition bath. If desired, the concentrate may be reconstituted by the addition of water either before entry to the bath or by adding water directly to the bath.

If there is present in the bath desirable materials which, because of their molecular size, are removed in the ultrafiltration process, these may likewise be returned to the bath either directly to the retained solute before entry to the bath, in the makeup feed as required, or independently.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable compositions which may be utilized within the scope of this invention. Virtually any water-soluble, water-dispersible or water-emulsifiable polyacid or polybasic resinous material can be electrodeposited and, if film-forming, provides coatings which may be suitable for certain purposes. Any such electrodepositable composition is included among those which can be employed in the present invention, even though the coating obtained might not be entirely satisfactory for certain specialized uses.

Presently, the most widely used electrodeposition vehicle resins are synthetic polycarboxylic acid resinous materials. Numerous such resins are described in US. Pat. Nos. 3,441,489, 3,422,044; 3,403,088; 3,369,983; and 3,366,563, which are incorporated by reference. These include a reaction product or adduct of the drying oil or semi-drying oil fatty acid ester with a dicarboxylic acid or anhydride. By drying oil or semi-drying oil fatty acid esters are meant esters of fatty acids which are or can be derived from drying oils or semi-drying oils, or from such sources as tall oil. Such fatty acids are characterized by containing at least a portion of polyunsaturated fatty acids. Preferably, the drying oil or semi-drying oil per se is employed.

Also included among such esters are those in which the esters themselves are modified with other acids, including saturated, unsaturated, or aromatic acids or an anhydride thereof. The acid-modified esters are made by transesterification of the ester, as by forming a di-or monoglyceride by alcoholysis, followed by esterification with the acid; they may also be obtained by reacting oil acids with a polyol and reacting the acid with the partial ester. In addition to blycerol, alcoholysis can be carried out using other polyols such as trimethylolpropane, pentaerythritol, sorbitol and the like. If desired, the esters can also be modified with monomers such as cyclopentadiene or styrene and the modified esters produced thereby can be utilized herein. Similarly, other esters of unsaturated fatty acids, for example, those prepared by the esterification of tall oil fatty acids with polyols, are also useful.

Also included within the terms of drying oil fatty acid esters" as set forth herein are alkyd resins prepared utilizing semi-drying or drying oils; esters of epoxides with such fatty acids, including esters of diglycidyl ethers of polyhydric compounds as well as other mono-, diand polyepoxides, semidrying or drying oil fatty acid esters of polyols, such as butanediol, trimethylolethane, trimethylol-propane, trimethylolhexane, pentaerythritol, and the like; and semi-drying or drying oil fatty acid esters of resinous polyols such as homopolymers of copolymers of unsaturated aliphatic alcohols, e.g., allyl alcohol or methallyl alcohol, including copolymers of such alcohols with styrene or other ethylenically unsaturated monomers or with non-oil modified alkyd resins containing free hydroxyl groups.

Any alpha, beta-ethylenically unsaturated dicarboxylic acid or anhydride can be employed to produce the reaction products described herein. These include such anhydrides as maleic anhydride, itaconic anhydride, and other similar anhydrides. Instead of the anhydride, there may also be used ethylenically unsaturated dicarboxylic acids which form anhydrides, for example, maleic acid or itaconic acid. These acids appear to function by first forming the anhydride. Fumaric acid, which does not form an anhydride, may also be utilized, although in many instances it requires more stringent than the unsaturated dicarboxylic acid anhydrides or acids which form such anhydrides. Mixtures of any of the above acids or anhydrides may also be utilized. Generally speaking, the anhydride or acid employed contains from 4 12 carbon atoms although longer chain compounds can be used if so desired.

While the reaction products can be comprised solely of adducts of the fatty acid ester and the dicarboxylic acid or anhydride, in many instances it is desirable to incorporate into the reaction product another ethylenically unsaturated monomer. The use of such monomer often produces films and coatings which are harder and more resistant to abrasion and which may have other similar desirable characteristics.

As shown in the art, it is preferred that in certain instances the neutralization reaction be carried out in such a manner that amido groups are attached to part of the carbonyl carbon atoms derived from the dicarboxylic acid or anhydride.

Compositions within this general class are described in U.S. Pat. Nos. 3,366,563 and 3,369,983.

Another vehicle comprises the fatty acid ester, unsaturated acid or anhydride reaction products and any additional unsaturated modifying materials (as described above) which are further reacted with the polyol.

Essentially any polyol can be employed, but diols are preferred. When higher polyols, such as trimethylol-propane, glycerol, pentaerythritol and the like are utilized, they are employed in small amounts, or in conjunction with the diol, or in the presence of a monohydric alcohol, and are used with adducts having a relatively low proportion of acidic component. Water-insoluble diols are often preferable, and especially desirable water-dispersed compositions for e'lectrodeposition are obtained using 2,2-bis (4-hydroxycyclohexyl)propane (which has given the best results), neopentyl glycol, l, l isopropylidene-bis(p-phenyleneoxy)di-Z-propanol, and similar diols.

The proportions of the polyol and ester-anhydride adduct which are employed depend upon various factors, but are in general limited only by the need to avoid gelation of the product. The total functionality of the reactants is a guide to determining the optimum proportions to be employed, and in most instances should not be greater than about 2.

In many instances, only part of the anhydride groups of the adduct, e.g., about percent, are reacted with the polyol. Of those anhydride groups reacted, it is preferred that only one of the carboxyl groups is esterified in each instance.

The product contains a substantial part of the original acidity derived from the dicarboxylic acid or anhydride; ordinarily the product should have an acid number of at least about 20. To provide a water-dispersed product, such as is used in electrodeposition processes, at least part of the remaining acidic groups are neutralized by reaction of the partially esterified product with a base.

The polyol reaction products and reaction conditions are more fully described in Application Ser. No. 450,205, filed Apr. 22, 1965, now US. Pat. No. 3,565,781 as well as the art cited above.

Another type of electrodepositable coating composition which gives desirable results are the water-dispersible coating compositions comprising at least partially neutralized interpolymlrs of hydroxyalkyl esters of unsaturated carboxylic acids, unsaturated carboxylic acids and at least one other ethylenically unsaturated monomer. These are employed in the composition along with an aminealdehyde condensation product, with the interpolymer usually making from about 50 percent to about 95 percent by weight of the resinous composition.

The acid monomer of the interpolymer is usually acrylic acid or methacrylic acid, but other ethylenically unsaturated monocarboxylic and dicarboxylic acids of up to about 6 carbon atoms can also be employed. The hydroxyalkyl ester is usually hydroxyethyl or hydroxypropyl acrylate or methacrylate, but also desirable are the barious hydroxyalkyl esters of the above acids having, for example, up to about 5 carbon atoms in the hydroxyalkyl radical. Mono-or diesters of the dicarboxylic acids mentioned are included. Ordinarily the acid and ester each comprise between about 1 percent and about 20 percent by weight of the interpolymer, with the remainder being made up of one or more other copolymerizable ethylenically unsaturated monomers. The most often used are the alkyl acrylates, such as ethyl acrylate; the alkyl methacrylates, such as methyl methacrylate; and the vinyl aromatic hydrocarbons, such as styrene, but others can be utilized.

The above interpolymer is at least partially neutralized by reaction with a base as described above; at least about 10 percent, and preferably 50 percent or more of the acidic groups are neutralized, and this can be carried out either before or after the incorporation of the interpolymer in the coating composition.

The amine-aldehyde condensation products included in these compositions are, for example, condensation products of melamine, benzoguanamine, or urea with formaldehyde, although other amine-containing amines and amides, including triazines, diazines, triazoles, guanadines, guanamines and alkyl and aryl-substituted derivatives of such compounds can be employed, as can other aldehydes, such as acetaldehyde. The alkylol groups of the products can be etherified by reaction with an alcohol, and the products utilized can be watersoluble'or organic solvent-soluble.

Electrodeposition compositions comprising the above interpolymers and an amine-aldehyde resin are more fully described in US. Pat. No. 3,403,088.

Still another electrodepositable composition of desirable properties comprises an alkyd-amine vehicle, that is, a vehicle containing an alkyd resin and an amine-aldehyde resin. A number of these are known in the art and may be employed. Preferred are water-dispersible alkyds such as those in which a conventional alkyd (such as a glyceryl phthalate resin), which may be modified with drying oil fatty acids, is made with a high acid number (e.g., 50 to and solubilized with ammonia or an amine, or those in which a surface-active agent, such as a polyalkylene glycol (e.g., Carbowax") is incorporated. High acid number alkyds are also made by employing a tricarboxylic acid, such as trimellitic acid or anhydride, along with a polyol in making the alkyd.

The above alkyds are combined with an amine-aldehyde resin, such as those described hereinabove. Preferred are water-soluble condensation products of melamine or a similar triazine with formaldehyde with subsequent reaction with an alkanol. An example of such a product is hexakis(methoxymethyl)melamine.

The alkyd-amine compositions are dispersed in water and they ordinarily contain from about l0 percent to about 50 percent by weight of amine resin based on the total resinous components.

Yet another electrodepositable composition of desirable properties comprises mixed esters of a resinous polyol. These resin esters comprise mixed esters of an unsaturated fatty acid adduct. Generally the polyols which are utilized with these resins are essentially any polyol having a molecular weight between about 500 and 5,000. Such resinous polyols include those resinous materials containing oxirane rings which can be opened in, prior to, or during the esterification reaction to provide an apparent hydroxy site. The vehicle resins are formed by reacting a portion of the hydroxyl groups of the polyol with the fatty acid, the ratio of the reactions being such that at least an average of one hydroxyl group per molecule of the polyol remains unreacted. The remaining functionality is then reacted with the unsaturated fatty acid adduct of an olefinically unsaturated dicarboxylic anhydride, such as maleic anhydride, this second esterification reaction being conducted under conditions so that esterification occurs through the anhydride ring, thereby introducing free acid groups into the molecule. Mixed acids of the class described are disclosed in Belgian Pat. No. 641,642, as well as in copending Application Ser. No. 568,l44, filed July 27, 1966 now abandoned.

In order to produce an electrodepositable composition, it is necessary to at least partially neutralize the acid groups present with a base in order to disperse the resin in the electrodeposition bath. Inorganic bases such as metal hydroxides, especially potassium hydroxide, can be used. There may likewise be used ammonia or organic bases, especially potassium hydroxide, can be used. There may likewise be used ammonia or organic bases, especially water-soluble amines, such as, for example, the mono-, diand tri-lower alkyl amines such as methylamine, ethylamine, propylamine, butylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine and m-methyl-butylamine, triethylamine, tributylamine, methyldiethylamine, dimethylbutylamine, and the like; cyclic amines such as morpholine, pyrrolidine, piperidine; diamines such as hydrazine, methylhydrazine, 2,3-toluene diamine, ethyl diamine and piperizine and substituted amines such as hydroxylamine, ethanolamine, diethanolarnine, butanolamine, hexanolamine and methyldiethanolamine, octanolamine, diglycolamine and other polyglycolamines, triethanolamine and methylethanolamine, n-amino-ethanolaming and methyldiethanolamine and polyamines such as diethylene triamine.

There may be present in the electrodepositable composition any of the conventional types of pigments employed in the art. There is often incorporated into the pigment composition a dispersing or surface-active agent. Usually the pigment and surface-active agent, if any, are ground together in a portion of the vehicle, or alone, to make a paste and this is blended with the vehicle to produce a coating composition.

In many instances, it is preferred to add to the bath in order to aid dispersibility, viscosity and/or film quality, a non-ionic modifier or solvent. Examples of such materials are aliphatic, naphthenic and aromatic hydrocarbons or mixtures of the same; monoand dialkyl ethers of glycols, pine oil and other solvents compatible with the resin system. The presently preferred modifier is 4-methoxy-4-methyl-pentanone-2 (Pent- Oxone).

There may also be included in the coating composition, if desired, additives such as antioxidants. For example, orthoampyphenol or cresol. It is especially advantageous to include such antioxidants in coating compositions which are used in baths which may be exposed to atmospheric oxygen at elevated temperatures and with agitation over extended periods of time.

Other additives which may be included in coating compositions, if desired, include, for example, wetting agents such as petroleum sulfonates, sulfated fatty amines, or their amides, esters of sodium isothionates, alkyl phenoxypolyethylene a1- kanols, or phosphate esters including ethoxylated alkylphenol phosphates. Other additives which may be employed including anti-foaming agents, suspending agents, bactericides, and the like.

In formulating the coating composition, ordinary tap water may be employed. However, such water may contain a relatively high level of metals and cations which, while not rendering the process inoperative, may result in variations of properties of the baths when used in electrodeposition. Thus, in common practice, deionized water, i.e., water from which free ions have been removed by the passage through ion exchange resins, is invariably used to make up coating compositions of the instant invention.

In addition to the electrodepositable vehicle resins described above, there may be present in the electrodepositable composition other resinous materials which are non-carboxylic acid materials. For example, as shown above, there may be added up to about 50 percent by weight of an aminealdehyde condensation product.

Other base-solubilized polyacids which may be employed as electrodeposition vehicles include those taught in U.S. Pat. No. 3,392165, which is incorporated therein by reference, wherein the acid groups rather than being solely polycarboxylic acid groups contain mineral acid groups such as phosphonic, sulfonic, sulfate and phosphate groups.

The process of the instant invention is equally applicable to cationic type vehicle resins, that is, polybases solubilized by means of an acid, for example, an amine-terminated polyamide or an acrylic polymer solutilized with acetic acid. Another case of such cationic polymers is described in copending application Ser. No. 772,366, filed Oct. 28, 1968 now abandoned.

In a manner similar to the anionic resins described above, the cationic resins may be formulated with adjuvants, such as pigments, solvents, surfactants, crosslinking resins, and the like.

The polyacids are anionic in nature and are dispersed or dissolved in water with alkaline materials such as amines or alkaline metal hydroxides and, when subjected to an electric current, they migrate to the anode. The polybasic resins, solubilized by acids, are cationic in character and when these resins are water-dispersed or solubilized with an acid such as acetic acid, the material deposits on the cathode under an electric current.

The invention is further described in conjunction with the following examples, which are to be considered illustrative rather than limiting. All parts and percentages in the examples and throughout the specification are by weight unless otherwise stated.

EXAMPLE I The vehicle resin in this example is an acrylic interpolymer comprising 55 percent butyl acrylate, 25 percent styrene, 5 percent hydroxyethyl acrylate and 15 percent methacrylic acid as a 74 percent solids solution in the mono-ethyl ether of ethylene glycol. The solution had a viscosity of 250,000 centipoises and an acid value of 72. The electrodepositable composition comprised:

Parts by Weight Ethoxymethoxymethyl melamine (XM-l l 16) 50.35 Vehicle resin (above 276.61 Diisopropanolamine 28.19

The above composition was reduced to 5 percent solids with deionized water.

Tin-free steel can stock panels were coated at 460 volts for 4 seconds at a bath temperature of F. The dry film thickness was 1.5 mils. The panels were rinsed immediately after removal from the bath with deionized water. A total of 600 parts of rinse containing 3.04 parts solids were added to 1,400 parts of the bath composition. This bath was subjected to selective filtration utilizing a Diaflow Membrane Ultrafilter PM-30 described above as Membrane A.

The bath was filtered by a batch process using membrane PM-30 at 50 psi. After removal of 600 parts of ultrafiltrate, panels as above were coated at 500 volts for 4 seconds at a bath temperature of 85 F., the dry film thickness being 1.5 mils.

The bath in its various stages had the following compositions:

The vehicle resin in this example is a maleinized tall oil fatty acid-adipic acid ester of a styrene-allyl alcohol copolymer of 1,100 molecular weight and 5 hydroxyl functionality comprising 39.8 percent of the copolymer, 52.9 percent tall oil fatty acids, 1.3 percent adipic acid and 6.0 percent maleic anhydride as a percent solution in 4-methoxy-4-methyl pentanone-Z having an intrinsic viscosity of 31,500 centipoises and an acid value of 41. The electrodepositable material had the following composition:

Non-Volatiles 1 100% Vehicle Non-volatiles 86.05%

20% maleinized linseed oil 5.05 Vehicle resin (above) 85.44 Allylether of methylolated phenol (Methylon 75108) 8.38 Cresylic acid 0.88 Surfactant (Witco 912) 0.25 Pigment 13.95%

Lead silicate 4.15 Manganese dioxide 8.03 Anthracite coal 83.40 Strontium chromate 4.15

Solubilizing amine 1/4 diethyl/triethylamine stituted by rinsing phosphatized steel panels coated from the electrodeposition bath in the usual manner with deionized water but in such a manner that the rinsing returned to the concentrate. Rinse water was added in this fashion until the solids were reconstituted to their original levels.

The properties or the starting and reconstituted bath materials were as follows:

Starting Reconstituted Bath Bath pH 8.50 8.65 Solids 11.3 10.0 Conductivity 75F, Mmhos/cm. 1740 1280 Conductivity adjusted to 11.3% solids 1450 MEQ'IIOO gm. total 8.68 7.31 Percent reduced 15.8 MEQ/IOO grns. solids 76.9 73.1 Percent reduced 4.9 PPM CQ 287 131 Percent CO reduction 54.3

MEG milliequivalents of amine Phosphatized steel panels coated from the reconstituted bath at 250 volts for 2 minutes, bath temperature 75 F., showed a slightly lower film build and had a slightly rough. appearance as compared to the original bath. This is due to solvent loss on ultrafiltration and can be adjusted by appropriate solvent addition to the bath.

EXAMPLE 111 The vehicle resin in this example is an Epon l004-ta1l oil fatty acid mixed partial ester with a maleinized tall oil fatty acid adduct comprising 45 percent Epon 1004, 48 percent tall oil fatty acid and 7 percent maleic anhydride, having an acid value of 59 and a viscosity of 230,000 at 85 percent solids in butyl Cellosolve. The electrodepositable material had the following composition at 12 percent solids:

Parts by Weight Vehicle resin solids (above) 195.3 20 percent maleinized linseed oil 129 Surfactant (Witco 912) Anthracite coal (Pigmentary) 2 Strontium chromate Basic lead silicate EH PPPP b-mqllnb 2,000 parts of the above composition were charged into an electrodeposition bath. The bath was subjected to ultrafiltration through a PM-30 membrane described above as Membrane A at 50 psi. After 4 hours, 1,000 parts of ultrafiltrate were collected.

Phosphatized steel panels were then coated from the electrodeposition bath at 250 volts for 2 minutes at a bath temperature of 75 F. The panels were rinsed with deionized water in a manner that the rinse-containing dragout returned to the electrodeposition bath. Fourteen panels were coated and rinsed in order to replace the ultrafiltrate.

The properties of the starting and reconstituted bath materials were as follows:

Recon- Starting Ultrastituted Bath filtrate Bath pH 7.7 7.7 7.8 Conductivity 75F,

Mmhos/cm. 2850 1600 2350 Percent solids 8.53 7.68 MEQ/lOO gm. solids 76.7 68.8'

Panels coated from the initial bath and the reconstituted bath, for example at 250 volts for 2 minutes at 75 F., were equivalent in appearance.

EXAMPLE IV The electrodepositable composition of this example and others similar thereto are described in Application Ser. No. 772,366, filed Oct. 31, 1968.

A reaction vessel was charged with 200 parts of an epoxy resin made from the reaction of epichlorohydrin and Bisphenol A, having an epoxide equivalent of 290 to 335 and a molecular weight of 580-670 Epon 836). There were added 58.5 parts of a monoalcohol derived from reacting 5 moles of ethylene oxide with one mole of ethanol using potassium hydroxide catalyst, 2.3 parts of stannous chloride and 28 parts of the dirnethyl ether of diethylene glycol, and the mixture was heated at l45-l50 C. for 3 hours. The modified epoxy compound obtained contained oxyalkylene groups and had an epoxide equivalent of about 890. Two hundred parts of the modified epoxy compound were heated to 70 C. with stirring and then 13 parts of 2-(beta-dimethylaminoethoxy)-4-methyl- 1,3,2-dioxaborinane were added over a 21-minute period, during which time the temperature rose to 92 C. After 5 minutes there were added 1,719 parts of deionized water, with stirring, along with sufficient formic acid to make the pH of the solution 4.4. There was obtained a colloidal dispersion having a non-volatile solids content .of 9.1 percent. This dispersion was electrodeposited using steel electrodes and the following conditions:

Bath temperature 70F.

pH 414 Deposition time seconds Voltage 225 volts Current 1.5-0.2 amp.

The electrocoated panels were rinsed with deionized water in a manner so that the rinse water was returned to the electrodcposition bath. A portion of the electrodeposition bath was subjected to ultrafiltration as described in Example 1 with sufficient ultrafiltrate being removed to return the percentage solids to the original level. Panels coated from the initial bath and reconstituted bath were equivalent in appearance and properties.

Other electrodepositable compositions, such as those hereinabove described, may be substituted for those exemplified. Likewise, various other filter or membrane means may be employed to obtain the improvements hereinabove described.

According to the provisions of the Patent Statutes, there are described above the invention and what are now considered its best embodiments; however, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically described.

We claim:

1. A method of reducing dragout loss on an article electrocoated in an electrodeposition bath comprising synthetic resin ionically dispersed in aqueous medium while controlling the composition of the bath which comprises rinsing an article with water in such a manner that the rinse containing dragout is returned to the electrodeposition bath and subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane retains said resin and passes water and solute of substantially lower molecular size than said resin and returning retentate from the ultrafiltration process to the electrodeposition bath.

2. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about p.s.i. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.

3. A method as in claim 2 wherein the pressure gradient is between about 25 and about 75 p.s.i.

4. A method as in claim 3 wherein the resin is a base-solubilized synthetic polycarboxylic acid.

5. A method as in claim 3 wherein the resin in an acid-solubilized synthetic polybasic resin.

# i i alt 

2. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and wherein the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.
 3. A method as in claim 2 wherein the pressure gradient is between about 25 and about 75 p.s.i.
 4. A method as in claim 3 wherein the resin is a base-solubilized synthetic polycarboxylic acid.
 5. A method as in claim 3 wherein the resin in an acid-solubilized synthetic polybasic resin. 